sea salt, black pepper
40g freshly grated Parmesan
40g semi-salted butter
Heat a 25cm cast-iron frying pan over a medium-low heat for 4-5 minutes. Whisk the egg whites until they are stiff, whisk the yolks in another large bowl with a pinch of salt and a grinding of black pepper. Fold the egg whites into the yolks in two goes, and then fold in the Parmesan.
Remove the pan from the heat, add the butter and swill until it is completely melted, then return the pan to the heat, tip in the omelette mixture and smooth the surface to even it out. Cook the omelette for 4-6 minutes until the underside is a deep crusty gold and the uncooked surface, if you touch it with the back of your finger, it feels lukewarm.
Use a palette knife to lift the edges of the omelette to see how it's doing. Loosen the omelette around the sides and carefully fold one half over, then slip it on to a large plate. Cut it in two and serve with the mousse that spills out from the inside spooned beside it.